Conventionally, some automobile engines are provided with an exhaust gas recirculation system, which introduces a part of an exhaust gas flowing within an exhaust pipe as exhaust recirculation gas (EGR gas) into an intake pipe, reduces the maximum temperature of combustion in the cylinders by mixing the EGR gas with intake air, and reduces harmful substances (for example, nitrogen oxides) contained in exhaust gas.
The exhaust gas recirculation system is provided with an EGR piping connecting an exhaust system and an intake system of the engine, and an EGR valve provided within the EGR piping. The EGR valve is capable of adjusting an opening degree. In other words, a reflow rate of the EGR gas is adjusted by adjusting the opening degree of the EGR valve.
In this kind of exhaust gas recirculation system, there is a possibility that deposit of a combustion product (oxides or carbides) is deposited within the EGR piping, for example, an exhaust gas reflow path formed within a cylindrical nozzle fitted into a valve housing. Since the deposit is generated from hydrocarbon (HC), carbon (C), oil or the like in the exhaust gas, and has a high viscosity, the deposit collects on an outer peripheral portion of the EGR valve, a drive shaft of the EGR valve, an inner wall surface of the exhaust gas reflow path and the like. Further, when deposit (sediment) collects on a portion between the outer peripheral portion of the EGR valve and the inner wall surface of the reflow path, or collects on a portion between the drive shaft and the inner wall surface of the reflow path, an opening and closing motion of the EGR valve is hindered, and it is impossible to well adjust the opening degree of the EGR valve. Accordingly, there is generated a problem that the EGR gas cannot be fed to the intake pipe, or the proper reflow rate of the EGR gas cannot be obtained. Particularly, in the case where a drive torque for opening and closing the EGR valve is small, or in the case where the opening degree of the EGR valve is controlled in a small angular range, the problem is particularly noticeable.
The following Patent Document 1 has been proposed as a structure for solving the problem. Patent Document 1 discloses a structure for opening and closing an EGR valve at a predetermined opening degree near a valve full-close position at a time when an engine stops. Accordingly, the collected deposit is scraped off by the EGR valve, and the sticking of the EGR valve is dissolved or prevented (hereinafter, this operation is called as “sticking avoiding operation”).
Further, the following Patent Document 2 discloses a structure for setting a motor speed low so as to increase a drive torque at a time of executing the “sticking avoiding operation”, in the structure carrying out the “sticking avoiding operation” by a stepping motor. Further, Patent Document 2 discloses a structure for starting the “sticking avoiding operation” mentioned above when an ignition switch is turned off, as an execution timing of the “sticking avoiding operation”.
However, in the structure which starts the “sticking avoiding operation” when the ignition switch is turned off such as disclosed in Patent Document 2 mentioned above, that is, in the structure which starts the “sticking avoiding operation” in conjunction only with the ignition switch, the following problem occurs.
The sediment of the deposit mentioned above is generated during the operation of the engine. Accordingly, when the engine has been running and stopped immediately before the ignition switch is turned off, it is possible to effectively remove the deposit collected during the operation of the engine, by starting the “sticking avoiding operation”. However, in the structure disclosed in Patent Document 2, even when the ignition switch is turned off from a state in which only an ON operation of the ignition switch is carried out, that is, a state in which a starter is not started (the engine is not started), the “sticking avoiding operation” is started. In this condition, the “sticking avoiding operation” is started in spite the fact that the deposit has not collected in the exhaust gas reflow path. In other words, a useless “sticking avoiding operation” is started. The “sticking avoiding operation” generally utilizes an electric motor adjusting the opening degree of the EGR valve. Accordingly, if the “sticking avoiding operation” is started when the ignition switch is turned off without the starter being started after the ON operation of the ignition switch is carried out, useless power consumption is caused. Further, when the ON/OFF operations of the ignition switch are carried out over a number of times, the “sticking avoiding operation” is started each time, and a dissipation of the electric power becomes great.
Further, in the “sticking avoiding operation” mentioned above, an operating sound of the electric motor or the like is generated. If a passenger such as a driver or the like hear the operating sound every time when the ON/OFF operation of the ignition switch (the ON/OFF operation which does not accompany the engine start) is carried out, the passenger feels disturbed.
Further, when the ON/OFF operation of the ignition switch (the operation turning off the ignition switch without starting the starter) is carried out in a condition that the engine is not operated over a long time, the “sticking avoiding operation” is started in a condition that the engine is cold. In the condition that the engine is cold as mentioned above, there is a possibility that the EGR valve is strongly stuck by the deposit which cannot be scraped off by the previous “sticking avoiding operation”. If the “sticking avoiding operation” is started under such a condition, the load of the electric motor is significantly increased, and there is a possibility that a service life of the motor is adversely affected.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-162665
Patent Document 2: Japanese Laid-Open Patent Publication No. 8-303307